The present invention relates to a video projector, and more particularly, to a video projector that allows a user to perform trapezoidal distortion correction while viewing an image projected onto a projection surface.
Video projectors are used in various types of facilities, such as home theaters, conference rooms, classrooms, training rooms, amusement halls, exhibition halls, and studios, to project images generated by a computer and the like or images recorded on a recording medium, such as an optical disc or a magnetic tape, onto a projection surface such as a screen.
A video projector is designed so that when arranged directly in front of a projection surface, that is, when an optical axis of the video projector is orthogonal to the projection surface, a rectangular projection image having the desired aspect ratio is projected on the projection surface with a size corresponding to the distance from the video projector to the projection surface (projection distance). However, since a video projector can be used in various locations, the video projector may not be arranged directly in front of the projection surface. Such a case results in diagonal projection and distorts the projected image. This distortion is referred to as trapezoidal distortion or Keystone distortion.
A typical video projector includes a display device (e.g., LCD panel), which modulates light from a light source. When the video projector is arranged directly in front of the projection surface, the display device is provided with an image signal that has been processed so that a rectangular projection image having a predetermined aspect ratio is shown with a size that is in accordance with the projection distance. Trapezoidal distortion caused by diagonal projection is normally corrected by correcting the image signal provided to the display device. Such correction is performed when it is difficult to change the location of the projector or screen.
Mainly two types of trapezoidal distortion correction are known in the prior art. The first type of correction is referred to as an aspect ratio-oriented mode, which is a trapezoidal distortion correction process that keeps the aspect ratio of a projection image constant (refer to Japanese Laid-Open Patent Publication No. 2005-123669). The aspect ratio-oriented mode is used when the adjustment direction is known. The aspect ratio-oriented mode is advantageous when the projector is misaligned with the projection surface in the upward direction or downward direction but not in a lateral direction (in this case, the diagonal direction extends in the vertical direction) or when the projector is misaligned with the projection surface in the lateral direction but not in the upward or downward direction (in this case, the diagonal direction extends in the horizontal direction). For example, if the video projector is used when diagonal projection occurs in the upward direction, the shape of the projected image expands in the vertical direction, and the lateral dimensions of the projected image increases at upper positions. When correcting such trapezoidal distortion, to maintain the desired aspect ratio of the projected image, the vertical dimensions of the projected image prior to correction are reduced by a uniform magnification. As for the horizontal direction, the plurality of horizontal lines forming the projected image prior to correction are reduced or enlarged in length by different magnifications. In this case, the trapezoidal distortion correction amount is directly determined in accordance with the diagonal projection angle.
The second type of correction is referred to as a trapezoidal distortion correction mode oriented to the image displayed on the projection surface (Japanese Laid-Open Patent Publication No. 2007-215029). In this process, the aspect ratio is not restrained in strict terms. Referring to FIGS. 5(a) to 5(d), a display device 1 such as an LCD panel includes a displayable region 2. An offset region 3 in which an image is not displayed is set around the displayable region 2. The region for a projected image is enlarged to fit into the entire displayable region 2. Then, a reference quadrangle pattern is formed by a frame, which extends along the periphery of the enlarged image region, or four corner portions 2a to 2d of the frame. The reference quadrangle pattern serves as an adjustment pattern image 4, which is for adjusting a projection area. The adjustment pattern image 4 is projected onto a projection surface 5 such as a screen and corrected to determine the projection area of the projection surface 5 and the display device 1 and obtain coordinates of the projection area on the display device 1. Deformation parameters are calculated based on the determined projection area, and the image signal is corrected with the calculated deformation parameters.